Heretofore, inorganic photoconductive materials such as selenium, cadmium sulfide and zinc oxide have been widely used in the photosensitive layers of the electrophotographic photoreceptors. However, selenium and cadmium sulfide are required to be recovered as toxic substances. Further, selenium is crystallized by heat and thus is inferior in the heat resistance. Cadmium sulfide and zinc oxide are inferior in the moisture resistance. Zinc oxide has a drawback that it is poor in the printing resistance. Under these circumstances, research efforts are still being made to develop novel photosensitive materials. Recently, studies on use of organic photoconductive materials for the photosensitive layers of the electrophotographic photoreceptors have been advanced, and some of them have materialized into practical use. The organic photoconductive materials have many advantages over the inorganic materials. For example, they are light in weight and easy to fabricate into films, and they can be easily manufactured into photoreceptors or into transparent photoreceptors depending upon the type of the material.
Recently, the main research activities are directed to so-called function-separated photoreceptors whereby functions of generating and transporting electric charge carriers are performed by separate compounds, since they are effective for high sensitivity, and organic photoreceptors of this type have been practically employed.
As a carrier transporting material, a polymer photoconductive compound such as polyvinyl carbazole may be employed. Otherwise, a low molecular weight photoconductive compound may be used as dispersed or dissolved in a binder polymer.
Particularly in the case of an organic low molecular weight photoconductive compound, it is possible to select as a binder a polymer excellent in the film-forming property, flexibility and adhesive property, whereby a photoreceptor excellent in the mechanical property can readily be obtained. For this purpose, not only hydrazone compounds and stilbene compounds but also triarylamine compounds have been studied (U.S. Pat. Nos. 3,180,730, 3,387,973, 4,123,269 and 4,127,412). However, it has been difficult to find a suitable compound for the preparation of a highly sensitive photoreceptor.
Especially, color copying machines have been developed in recent years, and photoreceptors suitable for such copying machines are desired. However, conventional photoreceptors are poor in the reproducibility of the cyan color, because the carrier transporting material of them has absorption of light in the cyan color region (400-500 nm).